1. Technical Field of the Invention
This invention relates to a pressure-sensing electric conductor used in a graphic input device and/or switching device of a computer system, and to the manufacturing method thereof.
2. Description of the Prior Art
A pressure-sensing electric conductor which becomes conductive when impressed by exterior force is widely used in prior art. FIG. 9 is a sectional view of an embodiment of the pressure-sensing electric conductor disclosed in Japanese Patent Laid Open Publication No. 897/1978. Said pressure-sensing electric conductor is obtained by filling the rubber 1, an elastic body, with metal particles 2, and then making admixture thereof. When pressure is applied on said conductor from the direction indicated by the arrow in the drawing, the metal particles located in the portion where pressure is applied contact each other to form a link of conductive substance from one end to the other. FIG. 10 is an embodiment of said pressure-sensing electric conductor applied to a tablet or switching device. The rubber 1 filled with metal particles 2 is interposed between a substrate 3 and a flexible protective sheet 4, and the electrodes E.sub.1 and E.sub.2 in stripes are formed on the inner side of the substrate 3. When pressure is applied in the direction shown by an arrow in the drawing, the impressed spot becomes conductive.
FIG. 11 is an embodiment of the pressure-sensing electric conductor of prior art, disclosed in Japanese Utility Model laid open under No. 41588/1981. Said pressure-sensing electric conductor is obtained by means of printing a pattern using the insulating ink 6 on an elastic electric conductive sheet 5. FIG. 12 shows said pressure-sensing electric conductor being applied to a tablet or a switch. When pressure is not applied on the conductor, the insulating ink 6 separates the electric conductive sheet 5 from the electrodes E.sub.1 .about.E.sub.4, and when pressure is impressed over the protective sheet 4, the electric conductive sheet 5 is deformed, and its portion where pressure is applied touches the electrode to become electrically conductive.
In the pressure-sensing electric conductor is generally provided by filling the elastic rubber 1 with metal particles 2 and forming an admixture thereof, or by printing a pattern of the insulating layer using the insulating ink 6 over the flexible electric conductive sheet 5. These pressure-sensing electric conductors may not provide satisfactory characteristics in the sensitivity, durability, and resolution because of the use of metal particles 2 or insulating ink 6.
As an alternative, an insulating layer made of resin and having a number of holes to be laminated over a sheet form of electric conductor is suggested. Said pressure-sensing sensing electric conductor having an insulating layer of resin, such as polyester mesh and the like, is not liable to deterioration over elapsed time, and has improved sensitivity characteristics and durability.
The pressure-sensing electric conductor using metal particles, as shown in FIGS. 9 and 10, generates varying resistance when conducting electricity because of the oxidizing of metal particles 2, giving cause to chattering at a spot where the conductor is applied with pressure because of a number of metal particles contacting each other, and thus causes the sensitivity to deteriorate over the elapsed time, and also results in costlier operation of a device it is applied with.
The pressure-sensing electric conductor, as shown in FIGS. 11 and 12, where a pattern of the insulating layer is printed using the insulating ink 6 over the flexible electric conductive sheet 5, the distances and thickness the insulating patterns formed by using said insulating ink 6 have certain limitations that preclude optional determination of the input sensitivity, and at the same time provide the durability against the repeated impressions limited to a certain extent.
These pressure-sensing electric conductors may not provide satisfactory characteristics in the sensitivity, durability, and resolution because of the use of metal particles 2 or insulating ink 6. The pressure-sensing electric conductors of prior art having the insulating layer of resin are produced by coating adhesive agent on the insulating substance of resin material, simply adhering it over a sheet form of electric conducting substance, and by pressing them together. The resulting conductors fail to attain either sufficient adhesive strength or satisfactory sensitivity characteristics.